The angle between the normal vector n of α plane and the normal vector m of β plane and the angle between the two planes;

Equality or complementarity. Plane included angle can only be acute, vector included angle is equal when it is acute, and complementary when it is obtuse.

The method you give can be determined with your right hand, such as (a 1, b2, c 1) and (a2, b2, c2) above. The finger passes through 2 first, then through 1, and the direction of thumb is the direction of the obtained vector. (This is called vector cross product in college linear algebra. )

The angle corresponding to the absolute value of the cosine of the included angle of the vector obtained by the vector method is the complementary angle of the line-plane angle;

The angle of the normal plane of the vector is given above. .